**Core Concept**
The Glasgow Coma Scale (GCS) is a neurological scale used to assess the level of consciousness in a patient. It consists of three components: eye opening, verbal response, and motor response, each scored separately to provide a total score.

**Why the Correct Answer is Right**
The total GCS score ranges from 3 (deep unconsciousness) to 15 (fully awake). The scores for eye opening, verbal response, and motor response are added together to give the total score. The total score is used to categorize patients into different levels of consciousness, with higher scores indicating better consciousness. The GCS is commonly used in emergency medicine and critical care to assess the severity of brain injury.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the GCS score is not measured in seconds or minutes. While some components of the GCS involve time intervals, the total score is a numerical value between 3 and 15.
**Option B:** This option is incorrect because the GCS score is not a percentage. The total score is a discrete value, not a percentage of a possible total.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the GCS score is not a perfect measure of brain function, and patients with similar scores can have varying levels of consciousness. Additionally, the GCS score can change over time as a patient's condition improves or deteriorates.

**Correct Answer: D. 15**